PURPOSE: To automatically and accurately discriminate the direction of a static grid and to prevent the occurrence of a moire pattern by applying the Fourier transform to picture signals for picture element trains of two directions and discriminating the direction where the picture element train having the smaller peak electric power is extended as the direction where a radiation absorbing material is extended.

CONSTITUTION: A radiation picture of a subject is photographed with use of a grid having many radiation absorbing materials extending in the fixed direction. Then the picture signals are obtained from a recording medium, then, the picture signals of the picture element trains of two directions which are approximately orthogonal to each other on the recording medium with either one of both trains set parallel to the direction of a static grid are converted by Fourier transform part 33 and 34 respectively. The peak detecting parts 35 and 36 obtain the peak electric power on a space frequency axis for the paired Fourier transformation data. Then the direction where one of two picture element trains that has the smaller peak electric power is discriminated as the direction of the static grid. Thus each block is shifted in the direction rectangular to the static grid direction in order to prevent the occurrence of a low frequency moire pattern.